Post Tagged with: "polymers"

Polymers from Elemental Sulfur

This post is contributed by John Spevacek, an industrial polymer chemist and the author of the blog “It’s the Rheo Thing

While organic chemists are familiar with the elements, very seldom do we ever make use of them as a reactant. Sure, we add elemental magnesium to Grignard reactions and we can add halogens/hydrogen across double bonds, but for the most part, the pure elements are oxidized or reduced or ionized or otherwise modified before they take part in our reactions.

The situation is even that much clearer for my field of polymer chemistry. Pure elements of any sort are just not used at all. We certainly don’t use elemental carbon and hydrogen to make polyolefins, and silicon wafers are useless for making silicone polymers. In short, the refined elements have no place in polymer chemistry.

Until now.

A recent paper in Nature Chemistry (pay-per-view/subscription) showed that elemental sulfur can be directly co-polymerized with an organic molecule. What was more surprising yet was that the polymerization occurred without the use of solvents or even initiators.

From my perspective as a polymer chemist, the uses of sulfur are limited and have historically fallen into three categories. First are the polymers that have the sulfur in the backbone, such as polyphenylene sulfide (PPS), polyethersulfone (PES), and all the countless thiol-ene polymers. Another class are the polymers where the sulfur is peripheral to the backbone, usually as a sulfonate group such as in polystyrene sulfate. And lastly, there are the elastomeric materials where sulfur compounds have been used to vulcanize (crosslink) the polymer chains.

What all three of these sulfur-containing polymers have in common, however, is that none of them are prepared from elemental sulfur. They all require either a reduced or an oxidized form of sulfur in order to form the polymers.

As implied above, this new reaction is very simple. The researchers merely melted the sulfur and added 1,3-diisopropenyl benzene (DIB) at ratios from 90/10 to 50/50 w/w. The S8 rings of sulfur opened up and copolymerized with the vinyl groups.

The reaction mechanism is not explicitly detailed, but I imagine it to be similar to what occurs in thiol-ene polymerizations. Since the organic comonomer is difunctional, the resulting product is crosslinked, not through the sulfur atoms, but instead through the organic monomer. The authors (with tongue-in-cheek) call this “inverse vulcanization”. However, despite the existence of this crosslinking, the polymer still flows as a thermoplastic. (Evidently the numerous sulfide bonds are breaking and reforming under the shear). This is fortunate as it allows the plastic to easily be shaped into a final product using conventional equipment.

While this is the only polymerization reaction I know of using a pure element, this discover by itself is interesting although somewhat limited. Working with molten sulfur imposes two big restraints on the choice of comonomers – that they first be soluble in the molten sulfur and more importantly, that they not volatilize upon exposure to the heat (185 C). In other words, this new reaction opens up only a small set of potential polymers.

But what properties this polymer is already showing!

Consider batteries. We are surrounded in our modern lives by lithium-ion batteries. They are in our cellphones and laptops, our cordless power tools, and even the Mars Curiosity Rover. A relative drawback of these batteries is that the anions are metallic and therefore heavy, reducing the energy density. It’s long been known that lithium-sulfur batteries have a high energy density and lower cost, but the degradation of the sulfur electrodes limits their long-term stability.

Preliminary testing of a lithium battery using this new sulfur-based polymer, however, shows that the performance is nearly identical to that of a standard lithium-sulfur battery but without the degradation. When this result is combined with the ease of processing this new polymer, the potential for lithium-sulfur batteries has suddenly become a lot sunnier.

Almost as sunny yellow as the color of elemental sulfur.

LeBron James Promotes Sheet-y Science

It’s been quite a year for the NBA All-Star: claiming his first NBA Championship, winning gold in the 2012 London Olympics, and now…promoting dietary supplements?

The product in question, Sheets®, offers variations on the “breath strips” made popular roughly a decade ago. Each strip contains different GRAS additives, such as melatonin to aid sleep, or caffeine in the Energy Sheets®. Despite the fecundity of the exclamation points in the FAQs, or even the curious swath of ‘beautiful people’ who promote this product, I’d be willing to give it a pass, if it weren’t for one teeny, tiny detail: the “Science page.”

Here’s the full scientific statement:

“It’s simple…Sheets® solve problems! Sheets® are paper-thin, individually wrapped pocket-sized strips. No cans. No bottles. Simply place on tongue and your problem dissolves. How? Sheets® are packed with nutrients/vitamins and other active ingredients that, when placed on tongue, will begin to dissolve allowing for easy digestion.

Hang on a second….AAAAAUGH!

OK, all better now. Let’s see if we can break that down further for our discerning audience. Apparently, the science of Sheets® involves dissolution (“place on tongue”) followed by digestion of nutrients/vitamins. Did everyone get/understand that, or should I repeat/rehash it again? Never mind those goofy pictures with the colorful stamped film, which looks uncomfortably like another orally administered molecule



Let’s go to our good friend Google patents to find some real science on this sheet-y product. I dug up two documents in short order: US patent 4,713,243 (Johnson & Johnson, 1987) and US 6,419,903 B1 (Colgate, 2002). Both patents describe various technologies for impregnating thin, extruded films of soluble polymer with medicaments for oral administration. Translation – edible drug strips.

The base polymer of choice, even 25 years ago, seemed to be hydroxyalkyl cellulose, one form of which we call pullulan. Various swell-able filler polymers, such as gelatin, corn starch, or PEG (polyethylene glycol) mix with the pullulan to regulate its toughness and stiffness, as well as to serve as a carrier for the active ingredients. For the Colgate breath strips, these include zinc compounds or alpha-ionone, which help to fight volatile sulfur compounds (VSCs) in your mouth. The J&J patent reaches even further, engineering strips to fight bacteria (sulfadiazine), pain (potassium nitrate), or to reduce swelling (hydrocortisone).

Honestly? I was most surprised by the level of formulation science that goes into each strip: viscosity tests, dispersion, dissolution, adherence, blending, and extrusion. Sounds like the perfect job for a p-chemist.

Just don’t get LeBron involved. Please.

Sandwiches, Gluttons and Picky Eaters

This post is contributed by John Spevacek, an industrial polymer chemist and the author of the blog “It’s the Rheo Thing

Quintus guest-blogged recently on that iconic sandwich molecule, ferrocene, an iron atom sandwiched between two cyclopentadiene rings. Ferrocene is the first discovered and best known of a broader class of molecules called metallocenes, molecules in which a metal atom is sandwiched between two aromatic ligands (not necessarily cyclopentadienes). The applications of ferrocene at present are rather limited, but that is not the case with metallocenes. I thought I would expand on this subject by showing the particular usefulness of these molecules – the metallocenes – to polymer chemistry. Most people, including chemists, have little idea how important these molecules are to their everyday life. The molecules themselves are not polymerized, but instead are catalysts for the polymerization of olefins such as ethylene and propylene.

 Before we can get into the reaction details, I first need to explain for the stereochemistry of polymers and why it is import. In a isotactic polymer, all the monomers have been added to the chain in the same orientation:

while in an atactic polymer, the orientation is random:

This stereochemistry is critical to the mechanical properties of a polymer. Atactic propylene is easy to make, but is a pile of goo that you can use as a pretty bad adhesive and not much else. The isotactic version however, can crystallize and that then builds the strength of the material. Crystalline polypropylene is a good strong material that we use every day in food packaging, dishwasher safe food containers, carpeting, nonwoven fabrics, ropes and hundreds of other uses.

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